"Polyesters" represented by a polyethylene terephthalate, a polybutylene terephthalate or a polyethylene naphthalate, are widely used for various uses, since they have excellent physical and chemical properties. And as for the uses, the polyesters are widely used not only as fibers for clothings and industrial fibers such as a tire cord by utilizing their excellent mechanical characteristics such as a strength and a modulus of elasticity, heat resistant characteristics, etc., but also as a film processed as a plane and a molded engineering-plastic product processed three dimensionally.
In general, the polyesters used in such various uses, are produced by a direct polymerization method or an ester interchange method. Here, firstly, the former direct polymerization method is described. In this method, a polyester precursor is produced by performing a direct esterification reaction of a dicarboxylic acid component composed of mainly an aromatic dicarboxylic acid and containing an alicyclic dicarboxylic acid or an aliphatic dicarboxylic acid, or their ester-forming derivatives with a diol component. and then performing a polycondensation of the above polyester precursor under an atmospheric pressure or a reduced pressure. While, in the later ester interchange method, a lower alkyl ester of the acid component with the diol component are subject to the ester interchange reaction to form another polyester precursor, and then a polycondensation reaction of the precursor is performed under the atmospheric pressure or the reduced pressure.
As to the above polyester polymerizations, conventionally, a batch-wise method is largely used, but in order to produce the polyesters at a low cost and to take an advantage of a scale merit, a shift to a continuous process has been promoted, and merits for adopting the continuous process such as improvements in the yield and quality of the product, a uniform quality, an improvement in operational efficiency, a reduction in production cost, etc., are extremely high.
In general, most of the continuous production methods of the polyesters are conducted by using a process combining each plural esterification reactors or ester interchanging reactors with polycondensation reactors. For example, it is performed that a monomer or an oligomer is formed by charging raw materials into the ester interchange reactor or the esterification reactor, then the formed material is consecutively supplied into an initial polycondensation reactor for performing the reaction under a reduced pressure to produce a polymer having a low molecular weight, and further the polymer having a low molecular weight is supplied to a polycondensation reactor under a reduced pressure to obtain a polymer having a medium molecular weight and a polymer having a high molecular weight.
In this case, a diol component distilled out of the polycondensation reactor for the polyesters, is condensed in a tube type heat exchanger or a wet type condenser generally in view of an economical advantage, and then recovered and reused as a part of the raw materials. Further, there are a method of recovering the diol component without a distillation and a method of recovering after the distillation in the recovery/reuse methods.
Incidentally, in the case of the former method, since water and low boiling materials as byproducts are contained in a large amount in the diol component distilled out of the polycondensation reactors, by using the diol component without the distillation, there are problems such as an effect to the physical properties of the obtained polyester, an inhibition to the ester interchange reaction and/or polycondensation reaction by the effect of impurities, or developing a difficulty in performing a stable operation. Therefore, the method of reusing the diol component through a distillation process is preferable to the method of recovering the diol component directly without the distillation.
On the other hand, in the later process, since the low boiling substances can be removed by the distillation operation, the above problems in the former process can be solved. Here, a distillation column becomes necessary in the later process separately, but since the difference in boiling points of the produced low boiling substances and the diol component is large, no big distillation facility is necessary and for example it is desirable to use a distillation column attached to the esterification reactor or the ester exchange reactor simultaneously for the distillation of the diol component.
Such a method for producing the polyesters and a device therefor, are effective in view of not requiring the installation of a new distillation device as an additional facility, and capable of reducing an operational cost and simplifying the facilities. For example, as such a method, in JP-A 60-163918 (hereinafter, JPA means "Japanese Unexamined Patent Publication"), a method for condensing a gas consisting mainly of ethylene glycol generated from a polycondensation reactor by a wet type condenser, supplying the condensed liquid to a distillation column installed at an esterification reactor to remove low boiling point impurities and then returning the liquid to a slurry mixing tank in a continuous production method of a polyester, was disclosed. And also in JP-1854847 (hereinafter JP means "Japanese Patent"), a polymerization device in which the side of an inlet port of a sealing liquid for a liquid sealing type vacuum pump in the vacuum pump installed for making an initial polymerization vessel as a vacuum, is joined to a piping for an ethylene glycol circulation liquid used as a coolant for each of the condensers of polymerization vessels in the later stage, and another polymerization device in which the ethylene glycol is recovered by joining the side of an outlet port of the liquid sealing type vacuum pump with a distillation column attached to an esterification reactor or an ester interchange reactor, with a piping, are disclosed.
However, in the production of the polyesters, the diol component distilled out of the polycondensation reaction stage contains impurities such as monomers and/or oligomers (hereinafter "the monomers and/or oligomers" are briefly called as "an adhering material"), etc., and in case that the adhering material is mingled in the vacuum system in the polycondensation reactor in accompanying with the diol component generated in the polycondensation stage, troubles are generated since they solidify at a low temperature. And if the diol component distilled out of the polycondensation reactor and recovered by a condenser is supplied to the distillation column as it is, the adhering material sticks and grows in the distillation column to reduce a separating efficiency in the distillation column, finally induce a trouble caused by a blocking and it becomes impossible to perform a stable distillation operation. Further, a similar blocking phenomenon is induced in the piping for supplying the diol component to the distillation column.
As a counter measure to solve the above problems, a method for removing the adhering material in advance by installing a cold trap can be cited, and by this method the adhering material can be discharged to the outside of the system. However, the amount of the adhering material discharged from the system increases in accordance with reinforcing a degree of a vacuum, a reaction temperature and an agitation of the reactants to increase the polymerization rate. The above increased amount is a serious problem in the initial polycondensation stage.
Thus, in JP-763609, a cooling collection device for the distilled out material equipped with an interfering plate was proposed. Also, in JP-998614, a method for keeping a valve for controlling the degree of a vacuum or the valve and its vicinity at .ltoreq.120.degree. C. and discharging the escaping martial by fluidizing with a hot water, a hot glycol, a steam, a glycol vapor or their mixture, was proposed. And further, in JP-A 49-126793, a polycondensation device having a jacket between the polycondensation reactor and the vacuum device and equipped with a foreign material removing device equipped with a baffle and a spraying nozzle for a washing at the inside thereof, was proposed. And in JP-1092979, a method for bringing the distillate into contact with a collecting liquid consisting of diethylene glycol and/or triethylene glycol between the polycondensation reactor and the vacuum generating device, was proposed.
Also, in the case of installing a filter in the cold trap aiming at the removal of the adhering material, since an increase in pressure loss and a blocking are induced by getting a clogging of the filter, it is necessary to take the adhering material out of the system continuously. However, the adhering material taken out of the system becomes a great loss unless it is subjected to be reused, but since the adhering material collected by the cold trap sticks firmly to the filter, etc., it is difficult to recover the material simply and at a low cost, and to furnish for the reuse.
In consideration of the above issues, the object of this invention is to eliminate troubles accompanying the blocking of the distillation column or the pipings caused by the adhering material and to enable the stable distillation operation in the continuous production of the polyesters by melt-polymerizing a raw material consisting of an acid component consisting of dicarboxylic acids composed of mainly an aromatic dicarboxylic acid and containing an alicyclic dicarboxylic acid or an aliphatic dicarboxylic acid, or the acid component consisting of their ester-forming derivatives and a diol component. And another object of this invention is to be able to recover the diol component without using a large-scaled distillation device, and to attain the reduction of operational expenses and the simplification of the facilities, and further to provide the method for the continuous production of the polyesters capable of reusing the separated/recovered adhering material as necessary.